1. Field of the Disclosure
Embodiments disclosed herein relate generally to a process for the separation and purification of benzene in a hydrocarbon stream. More specifically, embodiments disclosed herein relate to the selective hydrogenation of diolefins in a hydrocarbon stream, allowing for improved purification and separation of benzene.
2. Background
One common process long used by the refinery industry to upgrade raw naphtha to high octane gasoline is catalytic reforming. In catalytic reforming, the raw naphtha having a boiling range from about 46 to 177° C. (115° F.-350° F.) is passed over an alumina supported noble metal catalyst at elevated temperatures (about 493° C.-565° C. (920° F.-1050° F.)) and moderate pressure (about 2 bar to 39 bar (about 15-550 psig)). The catalyst “reforms” the molecular structures of the hydrocarbons contained in the raw naphtha by removing hydrogen and rearranging the structure of the molecules so as to improve the octane number of the naphtha.
Because of the multiplicity of the compounds in the raw naphtha, the actual reactions which occur in catalytic reforming are numerous. Many of the resulting products are aryl or aromatic compounds, all of which exhibit high octane numbers. The aryl compounds produced depend upon the starting materials which in a refinery are controlled by the boiling range of the naphtha used and the crude oil source. The “reformed” product from a catalytic reforming process is commonly called reformate and is often separated into two fractions by conventional distillations—a light reformate having a boiling range of about 46° C.-121° C. (about 115° F.-250° F.) and a heavy reformate having a boiling range of about 121° C.-177° C. (about 250° F.-350° F.). The aryl compounds in each fraction are thus dependent upon their boiling points. The lower boiling or lighter aryl compounds, e.g., benzene, toluene and xylenes, are contained in the light reformate and higher boiling aryl compounds are contained in the heavy reformate. In other circumstances, the light reformate may contain only the benzene, or only benzene and toluene, depending upon any downstream processing of the stream.
The demand for cleaner and safer transportation fuels is becoming greater every year. Two major sources of gasoline feedstock, including reforming and catalytic cracking, present both a problem meeting strict environmental regulations and impose certain health risks. For example, light reformate typically contains unacceptably high levels of benzene, a known carcinogen and an environmental contaminant. As such, refiners in the U.S and in other countries are required to remove benzene from reformate streams and other gasoline fractions. Refiners may also desire to remove benzene in order to produce the benzene as a product, due to the fact that benzene is valuable for use in chemical processing. For example, benzene is used as an industrial solvent and is also a precursor in the production of pharmaceuticals, plastics, synthetic rubber, and dyes.
U.S. Pat. No. 5,773,670 discloses a process for the hydrogenation of aromatics in a petroleum stream. However, like solvent extraction, the process is not selective to one aromatic compound. U.S. Pat. No. 5,856,602 discloses the hydrogenation of aromatics in a hydrocarbon stream utilizing a distillation column reactor wherein the placement of the catalyst bed and operation of the distillation column controls which aromatic is retained in the catalyst bed for hydrogenation.
The separation and purification of benzene is complicated by the presence of contaminants, such as diolefins. Olefins having more than one double bond (diolefins) have fewer uses than compounds such as ethylene or butane. The removal of diolefins is of value prior to the recovery of benzene since these compounds have been found to be detrimental in most processing, storage, and use of the benzene streams. Various options for the removal of diolefins from such streams may include extraction, hydrogenation, and alkylation. Diolefin hydrogenation may be conducted by contacting diolefins with hydrogen in the presence of a hydrogenation catalyst to convert the diolefins to olefins and paraffins. However, non-selective hydrogenation results in a reduced octane rating and thus diminishes the overall value of the fuel. Thus, selective hydrogenation of diolefins in a benzene-containing reformate is valuable in retaining a high octane rating while reducing diolefin concentrations.
After catalytic reforming of the raw naphtha, benzene may be separated from the lighter aryl compounds by extraction with any number of solvents and separated from other aromatics in the light reformate by distillation, typically using an aromatics extraction unit (AXU). Aromatics extraction is a very old process and typically separates a catalytic reformate or a coke oven light oil, via liquid-liquid extraction or extractive distillation or both, into aromatics and non-aromatics.
It is also conventional, in many catalytic reforming systems, to use a guard bed to provide clay treating of the hydrocarbon feed or the light reformate. A guard bed provides hot or cold clay treating using a fixed-bed, vapor-phase process to polymerize selectively unsaturated gum-forming constituents (i.e., diolefins). A guard bed may be used before or within an AXU and removes trace amounts, typically 10 to 5000 wt ppm, of diolefins present. These diolefins, if not removed, may cause the extracted benzene to fail acid wash color tests, e.g., ASTM D-848. Additionally, diolefins fed to an extraction unit may cause the loss of extraction solvent into the extract product (i.e., benzene), as is well known by those skilled in the aromatics extraction art. For nitrogen-containing extraction solvents, this may make the product benzene off-spec on nitrogen and may place additional load on the clay treaters to remove the lost solvent. However, clay treatment is not without its problems, including the raw material and disposal costs associated with the clay.
Accordingly, there is still a significant need in the art for economical methods to recover benzene from a reformate stream.